Engines sometimes use turbochargers to extract work from engine exhaust gas and transfer at least some of the extracted work to a compressor, which operates to increase engine boost pressure and, thus, engine power. Because one or more turbochargers can be associated with an engine, and turbocharger operation directly relates to gas and air pressures in the engine exhaust and intake systems, operation of the turbocharger itself can be affected by the engine operating conditions. When the engine operating conditions are changing with time, which are generally referred to as transient engine operating conditions, the various system pressures and air flow or exhaust flow rates may change, which will cause corresponding changes to the turbocharger input and output conditions, in this way affecting the efficiency of the turbocharger.
One particular operating condition of a turbocharger compressor that appears under certain operating conditions is compressor surge. A surge will typically occur in a compressor when a compressor impeller speed is reduced and air from a high pressure side or outlet side of the compressor is allowed to flow backwards through the compressor from the outlet side to the low pressure side or inlet of the compressor. Such conditions can appear, for example, when engine speed and load are reduced in a short period. Compressor surging may cause a further reduction in impeller speed, which is detrimental to compressor efficiency, especially if an increase in engine speed and impeller speed shortly follows the surge.
Certain engine applications that may normally and frequently experience substantial reductions in engine speed and load, for example, engines for stationary generators, marine applications, or earthmoving equipment, may be more prone to compressor surging than other engine applications. In one particular application such as a wheel loader, the engine turbocharger may surge numerous times during a loading operation, which includes using full engine load to push a loader bucket into a pile, and reducing engine load to about zero when the loader can no longer move into the pile. When a surge event occurs, engine fuelling or fuel command may drop from a large value to a small or zero value, i.e., the fueling or loading command drastically reduces. As the engine fuel or load reduces, a compressor pressure ratio, which expresses the pressure ratio of air at the outlet of the compressor over air pressure at the inlet of the compressor, begins to drop, as does the speed of the impeller of the compressor. Surge occurs when the compressor pressure ratio suddenly reverses its trend and spikes momentarily, before continuing to decline to a steady-state value.
Although compressor surging can cause physical damage to the compressor, even if it does not, it robs power from the compressor, which can result in lower compressor efficiencies and lower engine efficiency over a particular work cycle. Accordingly, compressor surging for certain engine applications operating under conditions favorable for compressor surging should be avoided or minimized. Various engine control strategies for reducing or avoiding surging have been proposed in the past. U.S. Pat. No. 6,954,047, for example, proposes increasing engine idle speed when a surge event is detected and, if no more than three surges are detected over an hour of engine operation, the engine speed is again reduced to a normal value. However, this, and other proposed solutions, may unnecessarily increase engine fuel consumption without conclusively avoiding a surge condition. Moreover, the resulting back flow of air from the engine intake system, which is at a higher pressure than a compressor inlet but that drops during a compressor surge, causes oscillations in the intake manifold pressure. These oscillations cause a further loss of engine performance and a surge or a “bark” noise to occur, which is noticeable to an operator.